There are actually very legitimate reasons why your buddies betrayed you in the final mission:
First of all, the majority of your betrayers are the buddies from the Northern district, whom you last saw hiding out in Mike's Bar under heavy fire from the faction you helped win the power contest in the region. They're under fire in the first place is because the winner faction declared them spies, which happens right after you eliminate the other faction's leader and outlived your usefulness. You are the reason why they're targeted in the first place.
After the ambush, we don't know what happened to your buddies, but presumably they were captured and tortured by that faction while you happened to be the one that miraculously got away almost unscathed and shortly went back to business as usual in the Southern district, up to an including working with the very faction that backstabbed you. Needless to say, this does not paint you in a very good light to your Northern distict buddies: why are you the only one who got away while they suffered painful torture under the hands of the enemy? What if you were the one that led the enemy to them in the first place? After all, you did play a Double Agent working for both factions, helping one side backstabbing another as if it's a normal Tuesday. There's a good reason to believe that sooner or later, you're going to sell them out, too. Thus, they decided to betray you first.
For the Southern region buddies, there will be two who would betray you. The first is your first best buddy who's Genre Savvy enough to realize that as you help trying to bring about peace to the two factions and stop the war, sooner or later they'll turn on the foreign mercs like them as they decided that they have outlived their usefulness (which is exactly what happened to you in the Northern district, which you forgot somehow), so they convinced you to take the diamonds that were supposed to bring about peace and use them as a ticket out of the country instead. Unfortunately for the both of you, the Jackal got the drop on you and stole the diamond briefcase, while leaving you to be the scapegoat as the foreign assassin who prevented the peace treaty. Once again, the factions turned on the mercs and you and your best buddy got captured and put into prison, where your best buddy tore you a new one for not going along with their plan (unaware about the whole Jackal business) before they got dragged away by guards, presumably to be tortured, while you managed to escape shortly afterwards. Same deal as the above example.
The second Southern buddy who went along with the betrayal is your next best buddy who replaced the above guy, and the one who's actually trying to talk down others from killing you. Presumably, this one guy actually didn't want to go along with the plan, but the former best buddy in the above example, who managed to escape capture somehow, convinced him to go along with it because of reasons provided. Since there are more people who hated you than liked you, the current best buddy gave in to peer pressure and joined their side.
Even after all these betrayals, you still continue to do work for both factions as normal, with a lot of potential backstabbing and violent rampages without any clear cause or reason, becoming a Walking Disaster Area. Not to mention, your last job has you working for the Jackal, the target you came here to kill, something that presumably other mercs realized as well. Now, why would you work for someone you're supposed to kill? Someone who's on an international most wanted list as a lethal killer and a merchant of death? Just what the hell are you doing to be working for someone like that? You've become a very dangerous and unpredictable beast, someone who's definitely not to be trusted and should be put down as soon as possible.
To sum up, the main reason why those buddies betrayed you is because you're a backstabbing bastard, whose actions playing two factions against each other ended up uniting them against the foreign mercs, putting big targets on their backs and trapping them in a wartorn country with no means to escape. Your actions put them at high risk while you suffered comparatively lightly because you apparently suffered Aesop Amnesia, which is not helped by you being unpreditable and very dangerous. Thus, they all have a good reason to turn against you, no matter how sympathetic or altruistic they initially were: self-preservation takes priority.
In the opening, as the battle of Pala rages around him, the protagonist equips himself with a pistol and a machete that the Jackal left behind during his surprise visit the night before. This foreshadows how he will takes up the Jackal's cause as his own at the end of the game. Even more symbolic in the machete's case: it's the only weapon that stays with you for the rest of the game (with one exception of the brief time you spent in prison in Act 2, where you quickly get it back), while guns come and go.
